The present invention relates to a casing position locator for detecting structural features of a borehole casing.
The borehole of a well, such as an hydrocarbon well, is often lined with a borehole casing which extends along the length of the borehole or sections of it. Borehole casings are generally formed of a series of individual tubes jointed together, e.g. by collars, to form a continuous borehole casing.
However, it is often necessary to perform maintenance, repair or modifications of the casing—either the joints or the respective tubes making up the casing. In order to do this, the relevant portion of the casing to be e.g. repaired of maintained must be accurately located. This is usually achieved by using a casing collar locator (CCL) to locate the respective collars.
The conventional casing collar locator (CCL) is a passive device generally comprising two permanent magnets 101, 102 and at least one detecting coil 103, as shown in e.g. FIG. 1. In use, the permanent magnets 101, 102 are arranged so that each magnet presents the same pole to the other—in the example shown in FIG. 1 each magnet is presenting its “south” pole to the other magnet. The detecting coil 103 for detecting a change in the local magnetic field is located between the magnets 101, 102. The detecting coil 103 is connected to a detector 104 for detecting an electromotive force (emf) induced in the coil 103. The amplitude of induced emf in the coil 103 is proportional to the rate of change of magnetic flux through it.
The conventional way to use such a CCL is to lower it down a borehole casing on a wireline (or slickline). The magnetic flux linking the magnets to the turns of the coil depends on the local magnetic properties of the casing. The flux is time-independent if the CCL is stationary—hence no emf is generated in the coil.
If the CCL is moved along the casing and there is no time-variation of the flux caused by a spatial variation of the casing's magnetic properties there is also no emf induced in the coil. If, however, there is a spatial variation of the casing's magnetic properties e.g. owing to the proximity of a casing collar, the flux will vary in time proportionally to the speed of the CCL along the casing, and a detectable emf will be induced in the coil when the tool is close to a collar.
Detecting the emf induced in the coil then provides an indication that e.g. a collar is present at that location.
When collars are located, the corresponding length of dispensed wireline is recorded, thereby giving an indication of the distance to each respective collar. The number and relative location of the detected casing joints are then compared with the well reference logs. An example of such a CCL is given in European patent application publication number EP-A-0697497.
However, a conventional CCL will also detect residual or permanent magnetization of parts of the casing structure, or surrounding environment, as it moves rapidly by them. Such detections may subsequently (and erroneously) be treated as a collar or some other structural feature of the borehole casing. Such spurious or specious detections can be costly and time-consuming. Furthermore, because of the use of permanent magnets, the conventional CCL is prone to attracting magnetic detritus. As the magnets cannot be readily de-energized, the detritus is difficult to remove.
Also, the magnetic permeability of materials such as steels depends on the stress and strain to which they are subject Residual magnetic field sensing for stress measurement (J W Wilson, G Y Tian, and S Barrans, Sensors and Actuators A, (2006)—electronic reference: doi10.1016/j.sna.2006.08.010). This will give rise to time-variations of the detected magnetic flux extra to the magnetic flux changes arising from the proximity of the collar. This will affect all methods that use quasi-static magnetic fields, whether arising from permanent magnets, the geomagnetic field, or the remanant magnetization of the casing.
The sensitivity of the conventional CCL is dependent on the speed at which it moves because it is sensitive to the rate of change of flux. In the past, rapid moving wireline tools have been used in conjunction with these conventional CCLs to e.g. maintain borehole casings, and so this has not proved problematic.
More recently however, it has been proposed to use alternatives to wireline tools, such as autonomous wellbore robots (AWRs), to e.g. maintain borehole casings. However, AWRs tend to move more slowly through borehole casings than wireline tools. Therefore, using a conventional CCL with a slow moving AWR is undesirable because the slow movement reduces the sensitivity of the conventional CCL. Indeed, the AWR may move so slowly that the sensitivity of the conventional CCL may prevent it from accurately indicating the location of e.g. casing collars.
EP-A-1302624 discloses another conventional CCL which replaces the sense coil with a giant magnetoresistive, sometimes referred to as a giant magnetoresistance, (GMR) digital field sensor, thereby eliminating the velocity dependence. However, GMR devices are sensitive to the absolute magnitude of local magnetic fields, and so a conventional GMR CCL is prone to providing specious indications as to the presence of a casing feature such as a collar.